Method of fabricating a radar reflector



Jan. 30, 1968 J. B. BRAuc-:R 3,365,790

v METHOD OF FABRICATING A RADAR REFLECTOR Original Filed June 18, 1965 4Sheets-Sheet l NVENTOR J. B. BRAUER METHOD OF FABRICATING A RADARREFLECTOR Jan.. 3o, 1968 4 Sheets-Sheet 2 A Original Filed June 18, 1965INVENTQR daJfP/f. 5164067? J. B. BRAUER METHOD 0F FABRICATING A RADARREFLECTOR Jan. 30, 1968 4 Sheets-Sheet 5 Original Filed June 18, 1965MEQ/1 INVENTOR. JOSEP/IE E BY ,1

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Jan. 30, 1968 J. B. BRAUER METHOD OF FABRCATING A RADAR REFLECTOR 4Sheets-Sheet 4 Original Filed June 18, 1963 INVENTOR.

'Un'ite States niPnent 3,365,790 METHD F FABRECA'IING A RADAR REFLECTURJoseph B. Brauer, 1309 Carroll St.,

Rome, NY. 13440 Original application .lune 18, 1963, Ser. No. 288,84).Divided and this application Dec. 9, 1965, Ser. No.

6 Claims. (Cl. 29-527) The invention described herein may bemanufactured and used by or for the United States Government forgovernmental purposes without payment to me of any royalty thereon.

This application is a division of my copending application Ser. No.288,840 filed June 18, 1963.

This invention relates to microwave reflectors, and more particularly tothe fabrication of isotropic microwave reflectors.

An isotropic microwave reflector is a reflector that reflects the waveback in the same direction as the incident wave regardless of thedirection of the incident wave. This will occur by using cornerreflectors which is the name commonly given to devices constructed withthree mutually perpendicular reflecting planes whose intersection lie ata common point about an axis about which the planes are equispaced.Incident electromagnetic energy entering the open face of the invertedpyramid formed by the planes is reflected from two planes of thisreector in such a manner that it is returned parallel to the incidentpath with no reduction in total incident energy save that due to qualityof the reflective surface.

The invention is based on the use of numerous individual cornerreflectors mounted with their open faces lying on the plane surfaces ofa regular polyhedra. Since the open face of the corner reflector formsan equilateral triangle on a plane normal to the axis of the reflector,there is the further requirement that the appropriate regular polyhedronhave equilateral triangles for external faces or other polygon surfaceswhich can be extended in such a manner to form triangular faces.

The ideal isotropic reflector should therefore consist of a regularpolyhedron with equilateral triangular faces and should approximate aspherical body, presenting a uniform array of individual cornerreflector to radiation incident from any direction. The relatively broadbeam or angle of incidence tolerance (deviation from the true axis) forradiation which the corner reflector will accept and return allows theuse of polyhedra with relatively few faces, only roughly approximating asphere.

An object of this invention is to provide for a convenient method offabrication of a microwave reflector of the type specified.

Other objects and features of the invention will more fully appear fromthe drawings in which:

FIG. l shows a single corner reflector;

FIG. 2 shows a front view of a solid sector of the polyhedron showing asingle equilateral triangular face further divided into four smallerequilateral triangular faces;

FIG. 3 shows an isotropic microwave reflector in the form of anoctahedron;

FIG. 4 shows a solid sector of the polyhedron in form of a pentagonwhich has been further divided into ve equilateral triangular faces;

FIG. 5 shows an isotropic microwave reflector in the form of polyhedronof multiple sides sufciently numerous to approach a sphericalconfiguration; the ligure also showing a method of anchorage t-o ground;

FIG. 6 shows the mold used to form a solid sector of the polyhedronshown in cross section along lines at 6 6 of FIG. 2;

ICC

FIG. 7 shows a mold designed to cast an isotropic microwave reflector inone piece;

FIG. 8 shows a method of Calibrating a ground mounted radar using aground mounted target and a method of serial calibration using a targetsuspended from a balloon;

FIGS. 9(a) and 9(b) show methods of mounting isotropic microwave targetsin an aircraft for better radar detection;

FIG. 10 shows a method of rotating the polyhedron to prevent easy radardetection; and

FIG. ll shows a method of using the microwave reflector for purposes ofcamouflage.

Referring now to the drawings in m-ore detail, FIG. l shows the basiccorner reflector made up of three mutually perpendicular right triangles11, 12, 13 joining at vertex 14 and forming equilateral triangle 15.

As shown in FIG. 2 each equilateral triangle 16 can be divided into foursmaller equilateral triangles 17, 1S, 19, and Ztl each in turn havingthe same properties as shown in FIG. l. This process of dividing theequilateral triangle can be continued in order to make the polyhedronmore closely approach a sphere thus reducing or eliminating slightvariations in returned radiation as a function of position of theincident beam.

The simplest form of a polyhedron suitable for use as an isotropicmicrowave reflector is the tetrahedron having four triangular faces.

FIG. 3 shows a Variation of the polyhedron in the form of an octahedronwhich has eight corner reflectors and eight equilateral triangularfaces. Four corner reflectors 28, 29, 30, 3l are shown. Construction ofthe reflector is shown with foam base S9 and reflecting surface 9i).

Other variations of the polyhedron are possible such as the dodecahedronwhich has twelve pentagon faces and as shown in FIG. 4; each pentagonface can be further divided into ve equilateral triangles 22, 23, 24,25, 26. Hence the dodecahedron would have sixty triangular faces.

FlG. 5 shows an isotropic microwave target 13) showing small cornerreflectors l5 sufficiently numerous to cause the polyhedron to thedesired surface. Construction is shown with foam base 131 and reflectingsurface 132.

There are many possible variations of simple or mixed polyhedron whichcan be used to achieve the desired size and distribution of individualcorner reflectors composing the isotropic reflector. These variationsmay be applied in order to match the measurements of the individualreflector to the frequency of radiation for which it is to be used byincreasing the total number of individual reflectors in a single face ofthe polyhedron or to make the polyhedron more closely approach a sphere,thus reducing or eliminating slight variations in returned radiation asa function of position of the incident beam.

Referring to FIG. 8, the isotropic microwave reflector can be used forcalibration of ground radar sets using a ground mounted target 57 todetermine microwave return and geographic location, thereby eliminatingprecise surveying of a reflector location and angular orientation. Theonly measurement needed would 'be distance from the radar set 5S. Tomake serial calibration of ground mounted radar, the isotropic microwavereflector 55 could be suspended from lballoon 56 sent up at a desireddistance from the radar set 53.

Radar pulses 51 transmitted, from antenna 52 strike the ground mountedmicrowave reflector S7 or the suspended reflector 55 and are reflectedback to the antenna with little loss of energy.

The invention can also be used to increase radar crosssection ofaircraft for easier and more precise detection by friendly radar in airand ground trallic control systems, or to cause asmall aircraft toappear much larger to an enemy, or to cause a single plane to appear asmore than one, generating confusion prior to or during attack. In FIG.9(41) small reiiectors oi and 62 can be mounted in the wing tips 63 and64 of aircraft 65, or in FIG. 9(b), reector 66, can be mounted on fueltank 67 of aircraft 69.

The reflector could be mounted so as to rotate in order to eliminate thehigh reiiection when desirable as shown in FIG. l(a). A polyhedron 71,is mounted on the structure 7?. of an aircraft in order to increasereflectivity. if it becomes necessary to eliminate this highreilectivity the jpolyhedron can be rotated 180 which would then exposera non-reflecting surface 73. This rotation can be accomplished bycausing rod '75 to move which is connected to rack M22 which turns gear'F7 which in turn is connected to the upper end of axle Si runningthrough polyhedron 7l. At the same time rod '78 is caused to move in theopposite direction as ro-d 7S by linking rod 7S to rod S2 by means oflever 83 turning at pivot point 8d which is attached to structure 72 bybracket SS and then causing rod SZ to move in the same direction as rod7S. Rod 78 is connected to rack 79 which turns gear Bti which isconnected to the lower end of axle 8i. With rods 75 and '78 moving inopposite directions, the polyhedron is caused to rotate. Rods F.75 and32. are actuated by electric current in coils 86 and S7 which encirclethe rods.

Referring to FlG. l0(b) which shows the switch assembly that controlscurrent ow in coils Se and 37, when button 93 mounted in panel 942 isdepressed, crossbar 95 joins contacts 3d and 97 thus closing the switch.Voltage source 93 causes electric current to flow in wire 99 which isconnected to coils S and 87 at the same time current is caused to flowin coil i60 which cncircles rod 101 that is connected to button 93. Rodlili and coil liti@ act as a solenoid thereby keeping the switch in theclosed position. Rod itil enters chamber itl-i which contains spring165. When button 93 is depressed spring i055 is compressed betweencollar F.0d and detent iti?.

In order to cause the polyhedron to reverse its rotation and againexpose the side of high reflectivity, switch iti?) which is normallyclosed is opened thereby breaking the circuit. Current ceases to ow incoil itl@ and spring 105 acting on collar lit causes rod itil, crossbarQ5, and button 93 to be raised to its original position. Current alsoceases to flow in wire 99 and coils and 87 at FIG. (51) and a spring notshown acting on axle 8i returns the polyhedron to its original positionof high redectivity.

The inventor can be used as a camouflaging device in order to deceiveenemy radar by using a complex of reectors to duplicate reflectingcharacteristics of existing buildings or landmarks or totallyobliterating a tarcet by saturating the target area with high returnreiiectors as shown in FiG. ll. Aircraft lll sends out radar signalsIii-2, M3, H4, and receives reiected signals H5, liti, H7. Althoughisotropic microwave reflect-ors HS and H9 are smaller than structure126, they will appear to be of equal size to the radar in aircraft lilldue to the high reectivity of the isotropic microwave reiiectors.

Another use is as an aid to investigation of various parameters ofmicrowave systems under development or test, such as field intensitydistribution.

The polyhedron can be fabricated either by casting solid sectors andthen assembling them or by casting the Aentire body of the rellector inone piece.

Where suliicient numbers of the reflectors are required to justifyhigher initial mold cast, a suitable mold, split to allow for removal ofthe cast part, could be designed to enclose the entire body of thereiiector and the body could be cast in one piece of foam plastic orother light weight material. Such a mold is shown in FiG. 7 which issplit at line 3 to allow it to be open for removal of the polyhedron.The mold is composed of drag assembly 39 and cope assembly It@ which arekept in alignment by guides 4i and 4Z which are press litted into dragassembly 39. The mold material can be poured. through feed line Whereonly a few reliectors of a particular size or configuration are requireda mold such as shown in FIG. 6 could be designed to enclose the volumecontained in solid sector i6 of the polyhedron. The solid sector maythen be formed by casting a low density from plastic or other materialin this mold cavity. The material is poured into the feed line 32 andthen into the mold 33 which includes drag assembly 34 and cope assembly35. Upper platen 37' is attached to shank .36 which is in turn attachedto ram of press not shown to insure proper pressure on mold 33.

The conductive coating required to reflect microwave radiation can beapplied in either of two manners. A hot metal spray coating can beapplied to mold over a mold release coating and the foam plastic or thermaterial would be subsequently introduced so as to bond the spraycoating during the casting operation. Another manner of coating wouldbe, after casting is completed, to spray, dip, or paint application of ametal paint, or other conductive coating which would adhere to the basematerial.

lf desired, the entire reiiector could be made of sheet metal as ahollow polyhedron or may be cast of a foamed metal such as aluminum ormagnesium for a more rugged structure requiring no separate reflectivecoating.

Weather proofing oi the reflector may be accomplished as a nal step inthe fabrication by coating with an epoxy or other weather-proof organicresin, or spraying on a thin layer of foam.

What I claim is:

l. A method of fabricating an isotropic microwave reiiector including amultiplicity of corner reflectors, said method comprising casting a lowdensity material into mold cavities designed to contain individualcorner reflectors, assembling castings of said corner reflectors to forma polyhedron, and spraying said poiyhedrcn with reflecting material toform said isotropic microwave reflector.

2. A method of fabricating an isotropic microwave reflector including amultiplicity of corner reflectors, said method comprising applying amold release coating to mold cavities designed to contain individualcorner retlectors, applying a hot metal spray over said mold releasecoatings, casting a low density material into said mold cavities andassembling castings of said corner reflectors to form said isotropicmicrowave reflector in shape of a polyhedron.

3. A method of fabricating an isotropic microwave reliector according toclaim i wherein said low density material is foam plastic.

A method of fabricating an isotropic microwave reflector according toclaim 2 wherein said low density material is foam plastic.

5. A method of fabricating an isotropic microwave retlector in the shapeof a polyhedron including a multiplicity of corner reflectors, saidmethod comprising applying a mold release coating to a mold cavitydesigned to contain the entire poiyhedron, applying a hot metal sprayover said mold release coating, and casting a low density material intosaid mold cavity.

6. A method o fabricating an isotropic microwave reiiector according toclaim 5 wherein said low density material is foam plastic.

References Cited UNITED STATES PATENTS 2,872,675 2/1959 Kennaugh 343-183,039,093 6/1962 Rockwood 343-18 3,103,662 9/1963 Gray et al. 343-18 X3,200,400 8/1965 Gill. 3,235,441 2/1966 George et al. 252-635 X J. L.CLiNE, Assis-ant Exmni/zer.

1. A METHOD OF FABRICATING AN ISOTROPIC MICROWAVE, REFLECTOR INCLUDING AMULTIPLICITY OF CORNER REFLECTORS, SAID METHOD OF COMPRISING CASTING ALOW DENSITY MATERIAL INTO MOLD CAVITITES DESIGNED TO CONTAIN INDIVIDUALCORNER REFLECTORS, ASSEMBLING CASTINGS OF SAID CORNER REFLECTORS TO FORMA POLYHEDRON, AND SPRAYING SAID POLYHEDRON WITH REFLECTING MATERIAL TOFORM SAID ISOTROPIC MICROWAVE REFLECTOR.